1. Field of the Invention
The present invention relates to amplifier devices. More particularly, it relates to an amplifier device which includes at least a driving amplifier circuit and a push-pull output circuit, the push-pull output circuit consisting of a first amplifier portion which turns "on" when the drive current of the driving amplifier circuit decreases, and a second amplifier portion which turns "on" when the drive current increases.
2. Description of the Prior Art
In the field of the amplifier devices of this type, many power amplifier devices recently developed have amplifier circuits which are directly coupled and employ the OTL system in order to put them into the form of integrated circuits. With such a construction, however, a great current flows through the push-pull output circuit. Therefore, a load short-circuit, for example, may result. Then, the power amplifier portions forming the push-pull output circuit fall outside the allowable operating ranges, and their constituent components are therefore destroyed.
In an example of a measure for solving such a problem, when the first and second power amplifier portions fall out of the allowable operating ranges, changes in the voltage and current on their output side are detected. The respective inputs of the first and second amplifier portions are adjusted on the basis of the detection, whereby the operations of the amplifier portions are controlled. The circuit thus far described is shown in a Japanese technical periodical "Denshi-Gijutsu (Electronics)," No. 2, Vol. 14, 1972, FIG. 6, in a paper entitled `Circuit Design viewed from Protecting System` on pp. 34 - 39.
With such a construction, the destruction of the transistors of the first and second amplifier portions due to overloads for the first and second amplifier portions, or due to an abnormal rise in the supply voltage, can be prevented. However, in the case of overloads, for example, the drive current at the output of the driving amplifier circuit becomes large. It is accordingly feared that the transistor constituting the driving amplifier circuit will fall into an overloaded state, leading to its destruction.
In order to solve the problem, a circuit has been proposed in which the input of the driving amplifier circuit is adjusted at the overload condition. With any of such constructions, however, protective circuits are so arranged as to separately control the input sides of the respective constituent parts. As a consequence, the required number of components is large, and the structure is complicated.
Besides, in the prior-art constructions, the transistors of the protective circuits disposed on the input sides of the first and second amplifier portions differ in type. For example, when a P-N-P transistor is used on the input side of the first amplifier portion, an N-P-N transistor is employed on the input side of the second amplifier portion. Accordingly, in the case where the power amplifier device of such construction is to be produced in the form of an integrated circuit, it is required to make either of them in the lateral structure and the other in the vertical structure. This is undesirable from the point of view of raising the degree of integration.